1. Field of the Invention
The present invention relates to an NMR analyzer.
2. Description of the Related Art
The NMR analyzer is an apparatus for evaluating and analyzing physical and chemical properties of a sample by placing the sample to be measured (to-be-measured sample) in a space of homogeneous static magnetic field, irradiating the to-be-measured sample with electromagnetic waves, and by utilizing the phenomenon of nuclear magnetic resonance that occurs.
A basic constitution of the NMR analyzer has been closely disclosed in a “Book of NMR”, Yoji Arata, Maruzen Co., 2000. In general, the NMR analyzer is constituted by at least a superconducting magnet for generating a static magnetic field, a probe for emitting electromagnetic waves onto a sample and for receiving freely induced decay signals emitted from the sample, a high-frequency power source for feeding a high-frequency current to the probe, an amplifier for amplifying freely induced decay signals, a detector for detecting signals, and an analyzer for analyzing the signals detected by the detector. The probe is chiefly a saddle-type or a cage-type probe coil, and usually has a function for irradiating the sample with electromagnetic waves and a function for receiving signals emitted from the sample. Further, a multi-layer air-core solenoid is used as the superconducting magnet to generate a magnetic field in the vertical direction. The superconducting magnet must be cooled with liquid helium and is, hence, contained in a low-temperature container called cryostat. The to-be-measured sample is inserted from the upper side of the room-temperature space penetrating through the cryostat up and down, and the probe is inserted from the lower side thereof.
At present, a method of analyzing organic matters by utilizing the nuclear magnetic resonance has been rapidly developing. Concretely speaking, organic compounds such as proteins having complex molecular structures can be efficiently analyzed for their structures on an atomic level by setting the resonance frequency of protons to be not lower than 500 MHz and by establishing a central magnetic field of not smaller than 11.5 T by using strong superconducting magnets. In this case, a highly homogeneous magnetic field of not larger than 0.1 ppm is required at a sample position near the center. As a practical product, an apparatus of 900 MHz having a magnetic field intensity of 21.1 T has been placed in the market, and still efforts have been made to develop an apparatus of 1 GHz having a magnetic field intensity of 23.5 T.
The NMR analyzer must measure the proteins having complex molecular structures maintaining good sensitivity, and has so far been developed by simply increasing the intensity of magnetic field in which the sample is placed without changing the fundamental constitution of the apparatus.
However, an increase in the sensitivity is accompanied by an increase in the size of the apparatus. For example, the superconducting magnet becomes higher than 5 m and heavier than 5 tons. The bulky superconducting magnet generates leaking magnetic field of as long as 10 m and must be installed in a dedicated building. Besides, the to-be-measured sample and the probe must be loaded at the center of the magnetic field. With the bulky apparatus, however, this operation becomes a burden. For example, the probe is inserted from the lower side of the cryostat and, hence, a space of as wide as 2 m is necessary for the insertion. This further makes it necessary to place the cryostat on a rack causing the center of gravity of the apparatus to become high and making it difficult to suppress its own vibration to a sufficient degree. Further, an enhanced ability of the superconducting wires necessitates the cooling with superconducting helium and, hence, requires cumbersome maintenance and an increased cost for the maintenance.
As for the effect for improving the sensitivity relying upon the shape of the probe coil, it has heretofore been known that the sensitivity can be improved by about 1.5 to about 3 times if a solenoid coil is used as the probe coil bringing about various advantages as compared to those of the saddle type or the cage type as disclosed in the above “Book of NMR” For example, advantage is obtained concerning easy control of impedance, filling factor and efficiency of RF magnetic field. With the superconducting magnet that generates electric field in the vertical direction, however, the high-frequency pulse magnetic field must be emitted to the sample in the horizontal direction. Therefore, it is practically difficult to wind the solenoid coil around the sample tube which is filled with a proteinaceous aqueous solution and is oriented in the vertical direction. Therefore, this superconducting magnet has not been generally used.